Variable capacity HVAC systems are typically controlled by electronic thermostats containing microprocessors which continuously monitor indoor air temperature and/or indoor air humidity. The thermostat temperature set point, and/or indoor air humidity set point is/are compared to the sensed or monitored temperature, or humidity value and the microprocessor in the thermostat evaluates a specific differential to generate a control signal.
To typically control a variable capacity compressor, as is found in some HVAC systems, the compressor speed and/or capacity are communicated to the thermostat over a communicating system bus via a software protocol. However, a thermostat containing a microprocessor configured to communicate commands and receive data over a system bus increases the cost of the HVAC system, which may create a barrier for users to enjoy the comfort and economic benefits of a variable capacity HVAC system.
Accordingly, there exists a need to control a variable speed compressor without the need for expensive communicating control, or the added features that a communicating control offers.